1. Field of the Invention
The present invention relates to fired iron-ore pellets and to a process for producing the same. More particularly, the invention relates to pellets for use in the production of pig iron in a blast furnace, wherein the pellets exhibit significant reducing properties as well as excellent softening and sticking properties over a high temperature range, i.e., the pellets exhibit excellent behavior and properties over a high temperature range.
2. Description of the Prior Art
The prior art iron-ore pellets which are charged into blast furnaces are classified into several types, i.e., acid pellets and self-fluxing pellets containing limestones and so on. These pellets are produced particularly in an attempt to increase the tumbler index and to improve the reducibility of pellets which characteristics are described in Japanese Industrial Standards (JIS M-8713), by improving the compressive strength and porosity of the pellets. However, the prior art pellets do not necessarily possess satisfactory behavior in their reduction characteristics at high temperature ranges in the lower part of a blast furnace (in the belly of the furnace and in the lower portions thereof), because of hindered gas flow into the inner portions of the pellets, which is attributable to the fact that the sizes of the pores in the pellets are less than 0.1 mm, and that the metallic iron in the outer peripheral portions of the pellets forms shell layers. As a result, the reduction of the pellets occurs reluctantly and low melting point slags are formed in the pellets, so that the pellets are softened, and the pellets stick to each other, resulting in various operational problems in a blast furnace.
Because of the increasing necessity to prevent air pollution, the amount of dust recovered from the various steps of iron manufacturing has increased because of the provision of highly efficient dust collectors. Recently, various kinds of dusts recovered from such processes have been reused. For instance, the dust from blast furnaces, converters and the like can be used as raw materials for pellets. These dust materials more or less contain carbon and are of a minute grain size so that they tend to produce micro-pores in the fired pellets. In addition, the dust materials also lower the compressive strength and reduction degree of the pellets, which standards are set by JIC, and are therefore not satisfactory as raw materials for pellets.
Furthermore, under a reducing atmosphere at high temperature ranges, metallic iron in the outer shells of the pellets provides a compacted layer therein, while the formation of the liquid-slag phase in the inner portion of the pellets is accelerated, thereby causing clogging of the pores in the pellets with the result that the reduction of the pellets is retarded. In addition, the pellets are of a spherical shape and contact each other in a surface-contacting relationship, and tend to contract to a considerable degree under a load, because of the formation of metallic iron and excluding of slag from the pellets, with the result that there arises a tendency to form large clogs of material in the furnace. Accordingly, the diffusion of gases into the interior regions of the pellets in a blast furnace is hindered, which causes an increased consumption of fuel.
In this manner, when the prior art fired pellets which are charged into a blast furnace descend into a high temperature zone, the reduction of the pellets does not readily occur, with the consequent acceleration of the softening and sticking phenomena of the pellets thereby forming large clogs of pellets. The clogs of the pellets result in a nonuniform flow of gas, hanging, slipping, broken tuyeres, and the like, all of which cause various problems in blast furnace operation.
A need, therefore, continues to exist for a manner in which the shortcomings of the prior art fired pellets can be improved.